1. Field of Invention
The present invention relates to mapping the presence and the location of the subsurface hydrocarbon reservoirs using the changes in the spectral and amplitude characteristics of the seismic pulse, due to elastic nonlinearity as the seismic pulse propagates through the porous and permeable reservoir rocks. The seismic wave or seismic pulse goes through a nonlinear transformation, and its spectral and amplitude characteristics change. Spectral broadening of the seismic pulse takes place as it is reflected, refracted, and transmitted through the porous reservoir rocks. Due to the elastic nonlinearity effects of the reservoir rocks new frequencies are generated. The generation of the new frequencies and their presence in the reflected signals is used to map the porous and permeable reservoir rocks. Reservoir rocks, which are highly permeable, also generate a Slow-Wave. The Slow-Wave travels at a much lower velocity than the Compressional Wave. The presence of the Slow-Wave also affects the amplitude and the spectrum of the reflected signals. The changes in the spectrum of the seismic reflected signals are used as an indicator to detect the presence of reservoir rocks and measure the reservoir rock properties. This invention would be extremely useful in detecting stratigrapic hydrocarbon traps that are difficult to map using structural information.
2. Description of the Prior Art
The current state-of-the-art seismic technologies that are being used to map the reservoir characteristics include 3-D seismic reflection surveys, seismic attribute analysis, signal amplitude extraction, and coherency techniques. In spite of all the recent progress in seismic data acquisition and seismic data processing, results are quite often non-unique and ambiguous and fail to identify the higher porosity and fractured zones that contain a significant portion of the hydrocarbon reserves.
New technologies and more sensitive methods of measuring the reservoir characteristics are to be developed and introduced to identify and map the higher porosity and fractured reservoir rocks, which may contain a large portion of the unproduced hydrocarbon reserves. In the past, the seismic industry has ignored the effects of elastic dynamic nonlinearity of the reservoir rocks. The measurement of the dynamic elastic nonlinearity of the reservoir rocks is a sensitive tool because the porosity induces an orders of magnitude change for the nonlinear coefficients and a few percent change for linear parameters of velocity, attenuation etc. [Reference: Donskoy, McKee, 1977; Paul Johnson, 1997].
The current seismic reflection methods use the response of the earth""s subsurface formations to the seismic waves for mapping the structural geology of the hydrocarbon reservoirs. For seismic reflection recording, the seismic source generates a seismic impulse, which propagates through the earth and the reflection response of the subsurface is recorded. The reflected signal characteristics depend on the acoustic and elastic properties of the rock formations. When the seismic wave encounters abrupt changes in the acoustic properties of the subsurface formations, it is reflected and refracted as it travels through the earth. The seismic measurements of the travel times and the amplitudes of the reflected signals define the subsurface geometry and provide an estimate of the acoustic impedances related to the subsurface rock velocities and densities. The seismic reflection record is basically the result of the convolution of the source-generated seismic pulse with the reflection coefficient series of the subsurface rock formations. The amplitude and the phase of the reflected and refracted signals are related to the elastic properties of different elastic mediums. During the downward propagation, there is a loss of higher frequencies, and the amplitude frequency response shows narrowing of the spectrum with depth, where the high frequency limit is imposed by the attenuation of the earth. Earth filtering effects are a major weakness of the current seismic methods.
The current seismic practice make two incorrect assumption when dealing with seismic wave propagation. First, they generally ignore the effects of elastic nonlinearity and treat sedimentary rocks as elastically linear. This is quite often to avoid extremely complex and cumbersome mathematics necessary when dealing with nonlinear behavior. Implicit in the assumption of linearity is the fact that the seismic wave or pulse recorded after being reflected and refracted can contain only those frequencies present in the input signalxe2x80x94the original seismic pulse that was initially transmitted. In the assumption of an elastically linear system no new frequencies can be generated.
The second incorrect assumption made is that the contribution of the Slow-Wave in the reflected and refracted signals from a porous and permeable rock formation is negligible and can be ignored. In reality, the reflected and refracted signals from a porous and permeable rock formation have two components. Part of the propagating energy is reflected and refracted from the rock matrix and part of the energy is reflected and refracted from the pore fluids that are contained in the rock formation. The compressional energy in the permeable rocks, which travels through the pore fluids, is known as a Slow-Wave. Due to the presence of this Slow-Wave in the permeable rock formation, the character and the amplitude of the reflected and refracted signals is affected and changed. The measurement of these changes provides us with a new seismic attribute that can be used to map the reservoir rock properties.
The current seismic methods assume that the earth is elastically linear, and the seismic wave or seismic pulse, as it travels through the earth subsurface formations, experiences no interaction between the frequencies generated by the seismic source. It is assumed that all the changes in the frequency spectra are caused due to attenuation, dispersion, and the reflection tuning effects. With the current assumptions of the earth being an elastically linear system, the recording equipment is designed and configured to handle the band limited seismic signals. Most seismic recordings for hydrocarbon exploration are made with a bandwidth of 6 to 8 Hz on the lower end of the seismic frequency spectrum and 70 to 80 Hz on the upper end of the frequency spectrum. The current recording practices are designed for a linear earth model. This limited frequency bandwidth is not adequate for recording the elastically nonlinear effects of the reservoir rocks. Elastic nonlinearity effects in a porous and permeable reservoir rock generate harmonics and sum-and-difference frequencies. These newly generated frequencies have to be preserved and recorded so that their presence can be detected and measured for mapping the porosity and permeability of the reservoir. To achieve that, seismic data has to be recorded with a bandwidth that has lower frequencies all the way down 0 Hz and higher frequencies to at least twice the frequency that is being currently used.
The current seismic bandwidth limitations are imposed by the recording characteristics of the receivers, amplifiers, and digitizers. These recording frequency bandwidth limitations are accepted because of the current assumption that the earth behaves linearly to the seismic signal. Higher frequencies are limited by the source generated signal and the earth""s attenuation. The limit on the lower frequency is quite often determined by the seismic source and the response of the receivers. However, if the earth model is modified to an elastically nonlinear system, then one will expect lower frequencies down to Zero Frequency signals being generated at the lower frequency end of the spectrum and the harmonics of the highest usable frequency being generated in the elastically nonlinear earth. At present, the recording equipment and the knowledge of using that equipment exists in the industry to modify the current systems, for accommodating the recording requirements of this invention. For this invention, the seismic data are recorded, with a broad frequency bandwidth that covers from Zero Frequency to a few hundred Hz.
In the past, geoscientists have noticed certain anomalies in the processed results of the seismic data, which are associated with the presence of hydrocarbons in the reservoirs. However, there is uncertainty and ambiguity in their interpretation because the anomalous behavior of the seismic results is neither well understood nor easily explained. As an example, low-frequency xe2x80x98shadowsxe2x80x99 have been associated with thin gas sands in Gulf of Mexico seismic data, without fully understanding what causes these shadows. These low-frequency shadows are sometimes not present when the gas sand is thicker. Also these shadows are stronger than the reservoir reflection and suggest that some unknown process not fully understood has generated these lower frequencies. The conventional linear earth models are unable to adequately explain this phenomenon.
The second example is the presence of very low frequency reflections, which are associated with the presence of oil. Such reflections have been recorded in the Soviet Union by Goloshubin et al. The explanations that have been given to justify their presence do not fit the realistic seismic wave behavior in the subsurface formations. This invention explains the cause of generating these anomalies. Once the mechanism, which generates these shadows and low-frequency reflections, is correctly understood, the geoscientist will be able to better utilize the seismic data for Direct Hydrocarbon Detection.
1) This Patent explains the spectral broadening of the seismic frequency spectrum due to elastic nonlinearity of the reservoir rocks. New lower and higher frequencies are created, which were not originally present in the input seismic pulse. Mapping these new frequencies, which are generated not by the source but by the reservoir rocks, provides a new method of mapping reservoir formations saturated with hydrocarbons.
2) The Patent further explains the generation of Slow-Wave in the permeable reservoir rock and its effect on the reflections and refractions from the upper and lower interface of the reservoir formation.
3) This invention shows that the generation of the Slow-Wave causes its reflection from the lower interface of the reservoir formation to be delayed and appear on the seismic reflection display as a xe2x80x98shadowxe2x80x99or xe2x80x98ghostxe2x80x99, which appears below the reservoir reflection event.
4) For the first time in the seismic industry, this Patent introduces a method of direct hydrocarbon detection that uses the frequency spectral broadening of the seismic pulse due to nonlinearity and the effects of Slow-Wave on the reflected and refracted seismic signal from the reservoir formations.
This invention provides a new method of mapping hydrocarbon reservoir formations that are porous and permeable. A conventional seismic impulse source is used to acquire the seismic reflection data. There is a major difference in this method from conventional seismic data acquisition currently being used. The difference is the requirement of recording seismic data using a wider frequency bandwidth of the seismic signals, compared to the normal practice in use today. To achieve the required broader frequency bandwidth, the seismic data are recorded using seismic receivers and recording instruments capable of preserving the signal frequency bandwidth from Zero Frequency to at least twice the usable high frequency expected for standard recording. Most of the seismic reflection signals for the current seismic hydrocarbon exploration are band limited at the higher part of the frequency spectrum to frequencies that are lower than 120 Hz. This high frequency limitation is accepted due to earth filtering and absorption effects, which severely attenuate the higher frequencies as the signal propagates downwards towards the deeper section of the earth. This invention requires extended bandwidth of up to 240 Hz so that the second harmonic of the highest usable frequency can be recorded. The newly created frequencies are not generated by the source, but are created in the subsurface reservoir rocks. At present, the necessary equipment and knowledge exists in the industry to record seismic data that will provide the extended bandwidth required for this invention.
This invention corrects the two incorrect assumptions accepted by the seismic industry when dealing with the seismic reflection mapping of the subsurface formations. The first incorrect assumption is that the earth is linear and that no new frequencies, not present in the source-generated signal, can be generated by the elastic nonlinearity of the earth. The second incorrect assumption is that the contribution of the Slow-Wave generated in the porous and permeable reservoir rocks can be ignored when dealing with the reflected and refracted seismic signals from a reservoir formation. This invention shows the significance of the spectral broadening due to the elastic nonlinearity of the porous formations, and the defines the contribution of the Slow-Wave generated in the reservoir in the reflected and refracted signals.
Two compressional waves, as they propagate through a porous rock that acts as an elastically nonlinear medium, interact with each other. Due to this interaction, the sum and difference frequencies of the two primary waves are created. These new frequencies constitute an xe2x80x98interactionxe2x80x99 wave that travels along with the primary waves. The amplitude of the summed frequencies or the xe2x80x98interactionxe2x80x99 wave is a function of the amplitudes of the two primary waves and the propagation distance though the nonlinear rock. Its amplitude grows with propagation distance due to nonlinearity but also decays with propagation distance due to earth""s attenuation. Reference U.S. Pat. No. 6,175,536 (Khan), where the interaction of the two crosswell seismic signals was successfully recorded as they propagate through the nonlinear reservoir formations.
In the case of a discrete frequency transmission through elastically nonlinear rock, the second harmonic is generated as a result of addition with itself, and Zero Frequency is generated as a result of differencing with itself. In the case of a seismic pulse, which is composed of many discrete frequencies, harmonics and sum and difference frequencies of all the discrete frequencies are generated. This results in the broadening of the frequency spectrum and creation of new frequencies, which were not present in the original pulse. The measurement of these new frequencies enables us to map the elastically nonlinear characteristics of the earth formations.
Biot (1956) proposed a comprehensive theory that explained many important features of the seismic wave propagation in fluid-saturated porous media. One of the important contributions of his theory is the prediction of a Slow Compressional Wave with a speed lower than that of the rock matrix or the pore fluid. The Slow-Wave involves a coupled motion between the fluid and the solid frame. The Slow-Wave""s velocity and attenuation depend on the morphology of the pore space and the pore interconnections, which also determine the fluid transport properties such as permeability.
When a pressure wave travels through a rock, the rock matrix and pore fluids are simultaneously compressed. The velocity of the Compressional Wave in the rock matrix is related to the mineral frame and the cementation between the grains, while the velocity of the slower component of the Compressional Wave that travels through the interconnected fluid path is determined by the physical properties of the pore fluids and the tortuosity of the connected pores in the rock. In the current seismic reflection recording and processing methods, the seismic industry has ignored the contribution of the Slow-Wave when dealing with the reflected and refracted seismic signals from a porous and permeable reservoir formation.
This invention deals with the effects of spectral broadening due to elastic nonlinearity and the effects of the Slow-Wave on the reflected and refracted seismic signals and how these nonlinear effects can be measured and used to map the reservoir rocks.
The spectrum of the seismic-reflected signal is derived from the initial characteristics of the source input pulse. The composite spectrum of the seismic signal may deviate from the input spectrum due to higher frequency attenuation as the seismic wave propagates through different subsurface formations. During the propagation of the seismic wave in the reservoir rocks, a certain amount of spectral broadening takes place. This spectral broadening is caused by the elastic nonlinearity of the reservoir rocks. The initial source-generated pulse is composed of multiple discrete frequencies. These multiple discrete frequencies generate their harmonics and additional sum and difference frequencies are created due to the nonlinear interaction between each other. This nonlinear interaction as the seismic wave travels through the reservoir formations creates new frequencies not originally present in the initial pulse. The amplitude of the harmonics and the sum and difference frequency components that create the broadening of the frequency spectrum, grows with the distance due to nonlinearity. However, the higher frequencies component of the new spectrum experiences a certain amount of decay due to the earth""s attenuation. The presence of these newly created frequencies in a seismic reflection representing a subsurface reservoir formation is an indicator that the particular formation is porous and permeable.
In spite of the limitations due to the band-limited nature of the seismic signals, the subtle changes in the wave shape or frequency spectrum can be used to interpret the subsurface stratigraphic details. Every seismic wave shape has a geologic significance, which only needs a definition. Changes in the spectral and amplitude characteristics of the seismic signal are associated with the porous and fluid-saturated rocks. This Patent establishes a physical link between the porosity and permeability of the reservoir rocks with the broadening of the frequency spectrum of the initial seismic pulse.
The frequency spectral broadening is a powerful seismic attribute to predict and map the hydrocarbon reservoir rocks that are of economic interest to the oil industry. Although geoscientists currently use seismic attributes to infer reservoir properties, the seismic nonlinear attributes have been ignored. The spectral broadening of the reflected signals from the top and bottom interface of the reservoir rocks is a powerful indicator of its rock properties.
There are a lot of different methods of frequency filtering and estimating spectral bandwidth of the seismic time varying signals, which are being used in the industry. There are conventional methods that use Fast Fourier Transform (FFT) for spectral estimation. For certain applications FFT is limited in its resolving power since it requires a fairly long spectral window. Discrete Fourier Transform (DFT) is preferred at times, since DFT offers greater speed and does not require similar transform length, as does FFT. Other time-frequency domain spectral analysis techniques have been developed, which can be used for spectral attribute extraction and are useful for estimating instantaneous frequency. The strength of this Patent is that it is not sensitive to different methods of spectral decomposition and filtering. For general applications, conventional Low-Pass and High-Pass filtering methods can be effectively used to identify and map the reservoir formations of interest.
To get full advantage of the method described in this Patent, seismic data should be recorded with broader frequency bandwidth than currently being used. It is important to record lower frequencies all the way down to Zero Frequency since lower frequencies survive earth attenuation better than higher frequencies. For recording higher frequencies the upper frequency bandwidth limitation should be high enough to record the second harmonic of the highest usable frequency in the reflected signal from the reservoir formation. Since the spectral broadening is taking place in the earth at the reservoir level, the spectral characteristics of the reflected signal do not go through the same attenuation process as the initial source generated pulse. For this Patent, the seismic data processing sequence is the same as for conventional data. The broadband frequency content and the true relative amplitudes are preserved. To get the desired seismic reflection image, the data are stacked, migrated, and displayed as a 2-D or 3-D volume. High-and low-cut frequency filters, which are designed to accommodate the source-generated input pulse, are used for standard display. In addition to the conventional display, two additional displays are generated. One is generated using a Low-Pass filter, which only allows the lower frequencies that were not present in the input pulse but were generated as a result of spectral broadening due to the elastic nonlinearity effects of the reservoir rocks. The second is generated using a High-Pass filter, which removes all the frequencies that represent the conventional reflection from the reservoir formation.
The two filtered displays of the 2-D or 3-D seismic reflection volume, which result from spectral broadening due to elastic nonlinearity, will highlight the nonlinear formations of the subsurface. Since there is a strong correlation between nonlinearity and reservoir porosity and permeability, the two high-and low-frequency displays will map the reservoir location and its extent.
When a pressure wave travels through a rock, the rock matrix and pore fluids are simultaneously compressed. The velocity of the Compressional Wave in the rock matrix is related to the mineral frame and the cementation between the grains, while the velocity of the slower component of the Compressional Wave that travels through the interconnected fluid path is determined by the physical properties of the pore fluids and the tortuosity of the connected pores in the rock.
In the published literature, the Compressional Wave that travels through the fluids in the interconnected pores is identified as Slow-Wave. Slow-Wave has been measured under laboratory conditions in samples of glass beads and different porous and permeable sandstones. The Slow-Wave travels at the fluid compressional velocity but does so over a longer distance along the tortuous interconnected pores between the two ends of the reservoir formation that is being measured.
When a seismic wave encounters an abrupt change in the elastic properties of different subsurface formations, part of the energy is reflected and part of it is refracted. The amplitude and the phase of the reflected and refracted signals are related to the elastic properties of the two subsurface formations. When one of the subsurface is porous, permeable and fluid saturated, its reflection coefficient becomes a function of the rock matrix and its pore fluids. Part of the reflected energy is reflected from the rock matrix and part of it from its fluid content. The composite reflection from a porous, permeable and fluid saturated formation is the result of the reflected energy from the rock matrix and its pore fluids. In most cases due to a large velocity difference between the faster Compressional Wave and Slow-Wave, the separate components of the reflected signal from the rock matrix and its pore fluids will be out of phase from each other. The character and the amplitude of the composite reflected signal will be modified according to the porosity and the permeability of the reservoir formation.
The faster Compressional Wave travels through the rock matrix and the Slow-Wave travels through the interconnected pores that are filled with fluid. Since there is a great deal of difference in the velocities of the Compressional Wave and the Slow-Wave, the refracted angles of the two waves become different. The two waves separate and travel through the reservoir formation on two independent paths at two different angles and at two different velocities. Two separate reflections occur at the lower interface of the reservoir formation. When the data are processed using conventional methods, an artifact, which represents the Slow-Wave reflection, occurs below the reservoir reflection, but delayed in time. At present this artifact cannot be explained and is known as a xe2x80x98shadowxe2x80x99 or xe2x80x98ghostxe2x80x99 in the industry. This Patent explains the cause of such a xe2x80x98shadowxe2x80x99 and how this information can be used to map certain reservoir properties. This Patent further uses the spectral broadening of the seismic-reflected signal for mapping the presence and location of the subsurface reservoir formations.
The amplitude and the velocity of the Slow-Wave are related to the rock tortuosity, permeability and pore fluid viscosity. The amplitude of the Slow-Wave is relatively larger in a reservoir formation, which has higher permeability and lower-viscosity pore fluids. Slow-Wave, which is created in the saturated reservoir rocks, travels at a much slower velocity compared to the Compressional Wave. The time delay of the Slow-Wave reflection from the lower interface of the reservoir formation, which appears as a xe2x80x98shadowxe2x80x99, can be used to calculate the Slow-Wave velocity. There are several other alternate methods of velocity analysis that are suitable for this application and are known and currently being used in the industry. Most of them measure, in one form or the other, the maximum coherency of the Common Depth Point (CDP) data by determining the best velocity fit for imaging the target geologic formations in depth or in time. Once the best root mean square (RMS) or migration velocity information has been derived, the interval velocities can be calculated. The alternate methods of determining Slow-Wave velocity can be used and the velocity further refined, after the presence of the xe2x80x98shadowxe2x80x99 has been established and the reservoir reflection identified. Once the velocity of the Slow-Wave has been determined, the tortuosity of the reservoir rock can be calculated. Knowing the tortuosity, the permeability of the rock can be estimated.
In summary, this Patent describes and explains the elastic nonlinear response of the porous and permeable reservoir formations to the seismic wave that propagates through it. For the first time this invention explains the anomalous behavior, which causes the presence of certain artifacts that have been noticed on the seismic reflection displays. The artifacts are directly related to the presence of hydrocarbons in the reservoir formations.